1. Field of the Invention
The present invention generally relates to IR (infrared) afocal lens assemblies and more specifically, to a dual field of view IR afocal assembly with substantially lens temperature independent performance.
2. Description of Prior Art
The purpose of an Afocal lens assembly is to collect collimated (plane wave) infra-red light, focus the rays in an image plane, and re-collimate the light through an exterior exit pupil. IR Afocals are in common use today in both military and industrial sensor systems. The design of each imager is constrained by the desired field of view, resolution, and size allocated by the system designer. One of the most widely used optical materials for use in IR optical design is germanium, which has a very high index of refraction (n=4.0), high optical transmission, and a relatively low color dispersion. For many compact IR sensor systems, germanium is one of the preferred materials that allows high performance with reduced complexity and affordable cost. Unfortunately, germanium also undergoes dramatic changes in transmission and index of refraction as temperature rises over 50.degree. C. Depending on the total amount of germanium in a given system, system performance can degrade rapidly in high temperature environments. The degradation includes a reduction in optical transmission due to the increased absorption, and a drastic change in optical power as the index of refraction changes with temperature.
While the prior art has reported using afocal lens assemblies none have established a basis for a specific assembly that is dedicated to the task of resolving the particular problem at hand. What is needed in this instance is a non-germanium infrared (IR) afocal lens assembly with substantially less temperature dependent performance.